The present invention relates to a test method for screwers and to devices applying this method. Powered screwers (of the electromechanical, pneumatic and other types) must give a precise torque wrench setting in use.
Therefore, availability of test devices is required which enable periodical checking of the torque wrench setting expressed by a screwer, so as to be able to establish whether adjustment, repair or replacement of those screwers that are no longer in conformity with specifications is required.
Precise standards exist giving regulations for measurement of screwer performances. For example, standard ISO 5393-81 describes a test method for evaluating pneumatic screwer performances and provides instructions on the statistical evaluation of measurements.
Obviously, since in normal use a screwer torque increases during rotation, any test for torque performances in a screwer must be conducted using simulators having a controlled torque gradient, so as to simulate the actual screwing down of a screw and enable the torque supplied by the screwer under a significant condition to be measured. For example, standard ISO 5393 presently requires that the tool should be tested under two limit conditions, in which the torque increase of 50% to 100% of the rated torque develops through a rotation of 360.degree. and 30.degree. respectively. In these value ranges (that in any case can be subjected to variations), standards ISO impose that the torque opposed by the simulator should increase according to a substantially linear law relative to rotation.
Simulators of the simplest conception are mere mechanical joint simulators. However, since the screwer calibration can be set to a variety of values, theoretically a variety of simulators should be provided or the simulator features should be susceptible of variation, which is generally rather complicated: therefore, mechanical simulators apply to the cases in which averaged out conditions are acceptable, but they do not exactly reflect features required by standards. In addition, the use of mechanical joint simulators has the drawback of requiring the screw to be unscrewed after each calibration so that a new test may be carried out.
In order to obviate the above drawbacks, as described in DE 33 05 457, test systems have been accomplished that use electromagnetic brakes of different types to simulate the torque-angle relationships required by standard ISO, by means of a modulation of the feed current to the brake coil.
The main defect in these systems is due to the fact that braking takes place by friction elements that, owing to their own nature, undergo feature variations as a result of heating and wear of the friction surfaces. Therefore a constant reliability overtime cannot be ensured.
In order to obviate the above inconvenience, the use of braking devices that do not operate by friction has been also mentioned, such as in the case in which current generators driven in rotation by the screwer are employed, which generators generally do not have appropriate electromechanical features.
In any case, there is a defect which is common to all the above devices, both those provided with traditional brakes and those involving current generators, i.e. the high moment of inertia of the braking unit which can be too high for quick screwers when calibrated to a low torque. In this case, the test cannot be carried out in that the static torque for driving the system in rotation exceeds the maximum torque for which the screwer is calibrated, so that the latter disconnects.
The general object of the present invention is to eliminate the above mentioned drawbacks by providing a test method and devices for screwers enabling specifications of the international standards to be accurately followed and constant results to be achieved even when screwers having particular features of low torque and/or high speed are concerned.